1. Field of the Invention
The present invention relates generally to optical communications, and particularly to group delay dispersion compensation for optical communications.
2. Technical Background
Wavelength division multiplexing (WDM) is commonly used to more efficiently utilize bandwidth for high-speed or high-bit-rate data transmission in an optical fiber network. In a WDM system, each high-speed data channel transmits the information contained within the channel at a pre-allocated wavelength on a single optical waveguide, such as an optical fiber. By sharing the transmission medium of the common waveguide, multiple high speed data signals can be multiplexed for transmitting data to a distant location. At the receiver end, channels of different wavelengths are separated or demultiplexed by narrow-band filters and then detected, optically monitored or otherwise used for further processing. As capacity needs increase, WDM systems multiplex higher and higher densities of wavelength channels to evolve into a dense WDM (DWDM) system by decreasing the channel spacings between channels. The International Telecommunications Union (ITU) has standardized a frequency grid for DWDM systems conformity. This ITU grid consists of 45 equally spaced apart frequency designations or channel spacing of 100 GHz which equates to a small free spectral range (FSR) of about 0.8 nm between adjacent channels in the wavelength range from 1533 to 1565 nm. As capacity is needed, commercial available systems may further evolve into higher number of channels more closely spaced.
To understand what the FSR is, a brief description of the optical signal is first given. The optical signal transmitted will typically be a series of light pulses of digital information with a xe2x80x9c1xe2x80x9d bit represented by the presence of light at a relatively high intensity or amplitude. Alternatively, a xe2x80x9c0xe2x80x9d bit corresponds to a substantial reduction in optical intensity or amplitude. Each optical pulse is a packet of waves where each wave in the packet is within a frequency bandwidth. Additionally, each wave in the packet is characterized by a plurality of different frequencies as well as a plurality of different amplitudes. An optical device (e.g. amplifier, repeater, filter, and fiber) has an amplitude response and a phase response. The amplitude response describes the attenuation of each frequency in the optical pulse after transmission through the optical device relative to their attenuation before transmission through the optical device.
The phase response determines the time delay for each frequency in the packet of waves. The period over which this phase or frequency response of the optical pulse repeats is called the free spectral range (FSR) of the optical device and can be written in frequency space in equation form as FSR=1/xcfx84 where xcfx84 is the propagation delay. For example, the FSR is the frequency spacing between two adjacent transmission or resonant peaks of a Fabry-Perot optical device.
In optical fiber communication systems chromatic dispersion is one of several key physical mechanisms that limit transmission capacity. Other limitations include propagation loss, polarization-mode dispersion and loss, and non-linear effects, such as cross-phase modulation (CPM), self-phase modulation (SPM) and four-wave mixing (FWM). As channel, spacing decreases as in DWDM systems, non-linear effects become more significant.
Chromatic or group velocity dispersion occurs when optical waves of different wavelengths (or spectral components of an optical signal pulse), propagate along an optical fiber at different group velocities to distort or broaden the pulse shape, limiting the rate at which information can be carried through the fiber. The linear dielectric response to the propagation of light in the dispersive medium or material of the optical device, typically silica from which optical fibers are typically made, for an optical communication systems, is dependent on the optical frequency (or wavelength). This material dispersion property of light propagation is one cause of chromatic or group delay dispersion, and is a consequence of the frequency dependence of the effective refractive index of the optical fiber. If the refractive index is a function of frequency, the effective refractive index will also be a function of wavelength. Hence, with positive group delay dispersion, the shorter wavelength light field experiences a higher refractive index and a lower group velocity, and therefore lags behind a longer wavelength light field.
Most optical devices used for transmitting optical pulses, such as single mode fibers, inherently apply a phase response to the optical pulse. This phase response changes the separation time between each frequency of the packet of waves, causing each frequency to be delayed for a different length of time. A given bit of a given (wavelength) channel propagates in the fiber at a speed determined by the group velocity. When each frequency of the packet of waves is delayed for a different length of time, the optical pulse output from such an optical device spreads, is broadened or otherwise distorted.
Relating to DWDM systems, another consequence of the wavelength dependence of the group delay is that it causes bits at different wavelengths to propagate at different speeds. As a result, bits from different channels initially overlapping in the time domain will eventually walk-off after sufficient propagation and acquire a relative group delay.
Hence, bits of a given channel will broaden because of group delay dispersion, and bits of adjacent channels will acquire a relative group delay. Broadening of the optical pulse is undesirable because, depending on the time between optical pulses, the leading and trailing edges of the broadened pulse can potentially overlap with the trailing edge of a previous optical pulse or the leading edge of a subsequent optical pulse, causing transmission or bit errors. Adjacent channels acquiring a group delay is desirable as this reduces the negative effects of non-linearities, such as FWM and SPM.
This broadening effect causes more bits to overlap at higher data rates in enhanced capacity systems. Once consecutive pulses have spread out so that they are no longer distinguishable, the information transmission limit of the optical communications system has been reached. This information transmission limit is expressed as a bandwidth distance product because it will be reached at a higher bit rate for a shorter optical communications link.
The dispersion of optical signals thus caused by dispersive devices can be reduced with a dispersion compensating element. Mathematically, the term dispersion refers to the first and higher order wavelength derivatives of the group delay experienced by the optical signal as it works its way through the device. The term group, delay refers to the slope of the phase response at each frequency in the packet of waves. The dispersion compensating element applies an additional dispersion to the optical signal which is the negative or opposite of the dispersion that was caused by the dispersive transmission fiber. This additional or second dispersion is added to the dispersion applied by the dispersive transmission fiber, so that the net system dispersion of the optical signal is about zero as seen in FIG. 8.
The two most widely used examples of fiber dispersion compensating elements are dispersion compensating (DC) fiber (or DCF) and dispersion compensating chirped fiber Bragg gratings (DCG). However, DCG""s have limited bandwidth, making it necessary to concatenate a number of gratings to compensate for light including many wavelength channels, such as a wavelength division multiplexed light. Having multiple gratings, the DCG""s are thus expensive to integrate into optical communication systems. Many chirped fiber gratings typically only compensate for quadratic dispersion, further limiting their utility to systems with quadratic dispersion.
Dispersion compensating fibers (DCF) are lossy. Lossy fibers are undesirable because they potentially reduce the optical power of signals transmitted along their length. DCF suffers not only from such high loss, but it also introduces extra power penalties due to optical non-linearities associated with its small effective area.
As mentioned already, transmission fiber itself introduces dispersion in an optical transmission system. Single mode fiber (SMF) fiber has its zero dispersion wavelength near 1.3 micron and its minimum loss near 1.55 micron. Thus transmission systems based on SMF fibers use a dispersion compensating element, possibly a DCF to compensate for dispersion.
To reduce the dispersion compensation requirements of the entire system, some WDM systems use dispersion shifted fiber (DSF) as the transmission fiber. Dispersion shifted fiber (DSF) can provide a transmission path with a close to zero dispersion at one wavelength near 1.55 micron, however, it suffers from certain nonlinearities, such as four-wave mixing (FWM), which affects signal integrity. Four-wave mixing is a nonlinear effect that causes a plurality of waves propagating down a fiber at predetermined channel spacings to create a new wave at a particular frequency. This newly created wave causes crosstalk when it interferes with other channels-within the ITU grid or signal channel plan. The magnitude and effects of FWM for a fixed amount of power per channel is inversely proportional to the frequency separation or spacing of the channels, the local chromatic dispersion of the dispersive transmission fiber, and the number of channels present.
At bit-rates in the high speed, high data 10-40 Gbits/s range, near the information transmission limit of high capacity DWDM systems, non-linearities become more significant a problem with conventional broadband dispersion compensating elements such as DC fiber. Thus, as bit rates increase, power per channel increases, and channel spacing decreases, to accommodate more system capacity, there is an increased need to compensate for chromatic dispersion without exhacerbating the undesired impairments caused by non-linear effects in the system.
One aspect of the present invention is the coupling of a negative compensation per channel filter to an optical receiving path for providing a discontinuous per-channel dispersion compensation in the optical receiving path without incurring or causing other types of system degradation such as loss. The optical receiving path includes an input port, at the input of the filter, for receiving an input optical pulse having a packet of waves at corresponding frequencies transmitted and received by an optical fiber.
In another aspect, the present invention includes a method for group delay dispersion compensation that exactly reduces to zero the total system group-delay-dispersion only over each channel bandwidth; in effect, enough system group-delay-dispersion is left between channels in order to avoid non-linearity-induced transmission impairments such as FWM. The steps include monitoring the individual group delay dispersion of an optical fiber transmission system at the center frequency of each ITU grid channel bandwidth. By only compensating for the individual group delay dispersion of the optical fiber transmission system at the center frequency of each ITU grid channel, a function results which does not compensate uniformly for group delay dispersion across the system bandwidth.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.